Patent Document No. 1: Japanese Unexamined Patent Publication (kokai) No. 2001-220630 discloses a Pb free Cu—Bi based sintered sliding material, in which 1 to 10% by weight of Bi phase is dispersed. Such intermetallic compounds as Ni—Si are present around the Bi-phase particles. Pb may also be present instead of or in addition to Bi. A sketch of the microscopic structure of a sintered material is shown in FIG. 1. The surface of the sliding material is finished by machining, and, subsequently, an overlay layer is deposited to provide a bearing.
Patent Document No. 2: Japanese Unexamined Patent Publication (kokai) No. 2005-350722, discloses a Pb-free Cu—Bi sintered sliding material, containing 1 to 30% Bi and 0.1 to 10% hard particles having an average particle diameter of 10 to 50 μm. The structure of the material is such that Bi phases finer than the hard particles are dispersed in the Cu matrix of the sintered sliding material. Microscopic photographs of the sintered material are shown in FIGS. 1-4. The surface of this Pb-free Cu—Bi based sintered material is paper-lapped to provide a ten-point average roughness of 1.0 μm.
Patent Document No. 3, WO 2008/018348, discloses a Pb-free Cu-based sintered sliding material, which contains 1 to 15% Sn, 1 to 15% Bi, and 1 to 10% hard particles having a particle diameter of 5 to 70 μm. All the hard particles are bonded to the copper matrix. As seen in FIGS. 1-4 of the microscope photographs, a number of Bi phases are dispersed in the sintered material.
The prior art publications mentioned above show, in their microscopic photographs and sketches, Bi phases, hard particles, a copper matrix and the like of sintered alloys. The materials are processed in successive steps, such as by cutting, rough polishing, fine polishing and buff polishing, thereby making the respective phases clearly apparent in the specimens prepared in this way for microscopic observation. Since Bi, Pb and the like are particularly soft, polishing is carefully carried out so that they are not shaved away nor flow them.
Conventional Cu—Pb based sliding materials have been recently replaced with Cu—Bi based sliding materials. In such a trend, Patent Document Nos. 1 through 3 focus on the characteristics of Bi from various points of view, including material cost, since Bi is more expensive than Pb, and hence the additive amount of Bi should be as low as possible.    Patent Document 1: Japanese Unexamined Patent Publication (kokai): No. 2001-220630    Patent Document 2: Japanese Unexamined Patent Publication (kokai) No. 2005-350722    Patent Document 3: WO 2008/018348